1. Field of the Invention
The present invention relates to a cathode material for use as a positive electrode in a secondary battery (that is to say, rechargeable battery), and relates in particular to a cathode material made with anhydrous ferric sulfate
2. Technical Background
Secondary batteries having a non-aqueous electrolyte solution, represented typically by lithium battery, comprises a negative electrode made of an anodic material of alkaline metal such as lithium, a positive electrode made of a cathodic material such as transition metal oxides and a non-aqueous electrolyte solution containing a salt of the alkaline metal of the cathode material. In secondary batteries of such a configuration, recharging is made possible by insertion or intercalation of the alkaline metal ions in the lattice structure of the cathode material. Therefore, the choice of a cathode material depends critically on its ability to act as a host to such Li insertion or Li intercalation.
The cathodic materials are conventionally transition metal oxides. On the other hand, in secondary batteries having vanadium pentaoxide or manganese dioxide as the cathode material, and lithium as the anode material, the open-circuit voltage is low and the charge/discharge current density is insufficient.
In recent years, there have been reports of reversible insertion of Li-ions at above 3.5 volts into such materials as: LiCoO.sub.2 with a layered rock-salt type structure (K. Mizushima, P. C. Jones, P. J. Wiseman and J. B. Goodenough, Mat. Res. Bull., vol. 15, 783(1980)); LiNiO.sub.2 (M. G. S. R. Thomas, W. I. F. David and J. B. Goodenough, Mat. Res. Bull., vol. 20, 1137(1985)); and LiMn.sub.2 O.sub.4 (T. Ohzuku, M. Kitagawa and T. Hirai, J. Electrochem. Soc., vol. 137, 769(1990)) of a spinel type structure. In 1991, lithium secondary batteries using LiCoO.sub.2 as the cathode material became commercially available (T. Nagaura and K. Tozawa, Prog. Batt. Solar Cells, vol. 9, 209(1990)). However, such batteries are expensive because they use expensive metals such as Co, Ni and Mn. This problem becomes even more acute for mass production applications.
Therefore, there have been various attempts at using low-cost iron compounds as the cathode material. For example, the following substances have been studied: FePS.sub.3 (A. Le Mehaute, G, Ouvrard, R. Brec and J. Rouxel, Mat. Res. Bull. vol. 12, 1191(1977); FeOCl (M. S. Whittingham, Prog. Solid State Chem., vol. 12, 41(1978); FeS.sub.2 (R. Brec and A. Dugast, Mat. Res. Bull., vol. 15, 619(1980); however, all of these materials suffer from low discharge voltage, and insufficient cyclability. On the other hand, Goodenough et al. published a study which compared the charge/discharge properties of monoclinic ferric sulfate, Fe.sub.2 (SO.sub.4).sub.3, made by refluxing ferrous sulfate of a composition given by Fe.sub.2 SO.sub.4.7H.sub.2 O, and of rhombohedral ferric sulfate, made by heating ferric sulfate of a composition given by Fe.sub.2 (SO.sub.4).sub.3.nH.sub.2 O, at 200.degree. C. (A. Manthiram and J. B. Goodenough, J. Power Sources, vol. 26, 403(1989)). Test cells made from the two polymorphs of Fe.sub.2 (SO.sub.4).sub.3 showed that, initially, they showed the same open-circuit voltage (OCV) of about 3.6 volts. However, the OCV for rhombohedral form of Fe.sub.2 (SO.sub.4).sub.3 exhibited a monotonically decreasing dependence on the molar Li content while the OCV for monoclinic form of Fe.sub.2 (SO.sub.4).sub.3 was independent of molar Li content up to about two moles. From their results, it was not clear whether or not discharging reactions in the test cells made from these materials are reversible. Furthermore, rhombohedral ferric sulfate is not a widely known material. The Joint Committee on Powder Diffraction Standards (JCPDS) reports the data on only two crystal structures for ferric sulfates: hexagonal and monoclinic. Therefore, the crystallography of the rhombohedral ferric sulfate reported in the study is uncertain.
The present inventors have made a detailed study of the crystallographic structure of ferric sulfate and of the effects of the water of crystallization on the charge/discharge properties (hereinafter referred to as cycling properties) of this material for use as a cathode material in secondary batteries.